This invention concerns a protective layer for carbonaceous materials, namely graphite electrodes, that is used to prevent lateral burn-offs during smelting in electric arc furnaces. The invention also concerns the method of producing this protective layer.
Metallic and ceramic protective layers, and also layers made of combined metal and ceramic, are well known. Their formulas change according to the desired characteristics of the protective layer. The metallic layers are used when the substrate is to be protected against corrosion or when the substrate surface is to be electrically conductive. The ceramic protective layers are used when high temperatures are involved or when abrasion is to be prevented. The combined metal-ceramic protective layers retain the characteristics of both the metallic and ceramic layers.
However, in some cases, the properties of the above named protective layers are not sufficient such as in the case of a graphite electrode used in electric arc furnaces when the layer is required to protect the base material against corrosion at high temperatures and at the same time provide for electric current feeding to the base material.
It is known that at more than 600.degree. C. there is an evident burn-off in graphite electrodes. The literature (e.g., Hutnik 1/1980, page 12) gives the properties of burn-off as follows: 0.7 kg/m.sup.2 at 600.degree. C.; 5.5 kg/m.sup.2 at 1000.degree. C.; and 10 kg/m.sup.2 at 1600.degree. C.
According to the German Pat. No. 1271 007/, it is possible to use a protective layer containing aluminum, silicon carbide and other heat resistant materials.
German Pat. No. 1,671,065 provides protective layers consisting of a basic layer formed mainly of silicon and a top layer containing mostly aluminum. These layers are applied by flame spraying.
According to German Patent Application No. 2,722,438, it is also well known to provide a protective layer where there is a fiber interlayer between the basic and top layers according to the German Pat. No. 1,671,065.
In the Soviet authorship No. 827 460, a protective layer is mentioned made of a composition of TiB.sub.2 and water glass applied on the electrode and then for 3-10 minutes processes by a plasma fusion at 3000.degree.-6000.degree. C., anode voltage 9-10 kV and anode current 3.8-5 A, while the plasma flame is 80-800 mm long.
The Czechoslovakia AC (Authorized Certificate) 217 720 presents a protective layer based on oxide ceramics and metal filler, e.g., copper or nickel.
The British Pat. No. 1,419,302 and the Bulgarian AC No. 11029 describe a production method for a protective layer on carbonaceous products, namely on electrodes. First, the aluminum layer is metallized on the products and then at normal heat, e.g., with a metal-spraying gun, a paste of aluminum, silicon carbide, titanium dioxide and boric acid is sprayed over and baked by electric arc; then comes the second metallizing with a second layer of paste and the second baking by electric arc. Then this layer is metallized with aluminum again, a graphite layer is applied and baked over, and then the product is polished.
The layers which are known so far are showing lower adhesion to graphite, especially at more than 800.degree. C., when heated and cooled in alternating cycles. Often cracks appear and the layer starts peeling off. Sometimes the layers peel off during the storage or electrodes. Some layers, as well as some methods of production, are rather complicated and demanding in production, and this are economically undesirable. In some protective layers there occurs a change of resistivity during storage.